The transcription of eukaryotic genes is regulated by specific DNA binding proteins that assemble on cis-acting DNA sequences in both promoters and enhancers (reviewed in Maniatis et al., 1987; Jones et al., 1988; Ptashne, 1988; Struhl, 1989). Many of these DNA binding proteins are ubiquitous in their expression and probably serve a general role in gene transcription. Others are restricted in their expression to one or a few cell types. Several examples have been documented in which the expression of this latter type of DNA binding protein correlates with expression of genes to which they bind (Staudt et al., 1988; Muller et al., 1988; Scheidereit et al., 1988; Bodner et al., 1988; Frain et al., 1989). The biochemical characterization of these cell type specific activators of gene expression will provide a critical step in our understanding of gene regulation.
Recently, a novel genetic approach was used to isolate a variant of simian virus 40 (SV40) in which growth was restricted to CV-1 cells and lymphoid cells (Pettersson and Schaffner, 1987). This variant was selected from an SV40 construction that lacked the 72 bp enhancer region. Characterization of this variant revealed that a duplicate region containing a purine rich sequence called the PU box (5'-GAGGAA-3') was able to substitute for the deleted wild type enhancer. A nuclear activity present only in T-cells and B-cells bound to the PU box. However, a specific protein was not shown to be responsible for the activation of transcription through the PU box. A PU box was independently confirmed in both human and mouse T-cells associated with expression of IL-2 (Serfling et al.; Shaw et al.). While it was shown that mutation in the PU box resulted in up to an 85% reduction in transcription and a factor designated NFAT-1 bound the PU box, it was not shown that NFAT-1 was responsible for the activation of transcription through the PU box.
The present invention describes a DNA binding protein, PU.1, that may be related to this lymphoid specific enhancer activity. The PU.1 protein binds to the PU box, is a transcriptional activator and is expressed in macrophages and B cells. Importantly, since PU.1 is not present in T-cells it represents a novel protein and demonstrates, for the first time, a family of transcriptional activators, expressed in different cell types which recognize the PU box. Thus, this invention satisfies a critical need to find cell specific transcription regulators and relationships between these regulators, especially in those cells which mediate the immune response.